diff -u -r a/CMakeLists.txt b/CMakeLists.txt
--- a/CMakeLists.txt	2020-11-23 08:59:08.000000000 -0600
+++ b/CMakeLists.txt	2021-01-15 17:15:43.665745800 -0600
@@ -271,7 +271,11 @@
 if(SIZEOF_SSIZE_T STREQUAL "")
   # ssize_t is a signed type in POSIX storing at least -1.
   # Set it to "int" to match the behavior of AC_TYPE_SSIZE_T (autotools).
-  set(ssize_t int)
+  if(WIN32 AND CMAKE_SIZEOF_VOID_P EQUAL 8)
+    set(ssize_t ptrdiff_t)
+  else()
+    set(ssize_t int)
+  endif()
 endif()
 # AC_TYPE_UINT8_T
 # AC_TYPE_UINT16_T
